The invention relates to an automatic machine for shaping the external profile of lenses fitted to spectacles.
There are two types of shatterproof organic material in current use for spectacle lenses, namely CR 39, and polycarbonate, which differ substantially in terms of hardness; polycarbonate lens material is markedly tougher to work than CR 39, and hitherto, has required the use of grinding rather than cutting equipment.
Shatterproof lenses are supplied in circular format with a diameter significantly greater than the ultimate dimensional requirements, in order to permit of being cut down to the size of any given model of spectacle frames.
Lenses of the type have to be given a bevel, which can be either positive (in relief) or negative (sunken), in order to enable their fitment to the spectacle frames. Conventional frames require three different types of bevel, two of which in relief, the other sunken, which must be located at a given distance from the outer edge of the lens, according to the type of frames and the thickness of the lens itself.
Machines currently adopted for shaping the external profile of spectacle lenses are provided with a rotatable shaft carrying a cutter or grinding wheel, according to whether CR 39 or polycarbonate material is to be worked. The lens is held between coaxial pivots and rotated, the pivots themselves being mounted to a hinged support that can be drawn nearer to or distanced from the cutter or grinding wheel by hand, following the outline dictated by the pattern lens.
Conventional machines are beset by a number of drawbacks over and above the simple fact of being manually operated.
A first drawback is that the cutter (or the grinding wheel) must be changed each time different bevel requirements are encountered; also, the lens carriage is fed manually into the cut, the result being that cutting pressure is not steady and even, but subject entirely to the skill of the operator.
In conventional machines, there is no way that the position of the bevel, in relation to the outer edge of the lens, can be adjusted according to the thickness of the lens; neither is there any automatic method of altering the position of the lens during cutting so as to adapt to its curvature and thus ensure correct location of the bevel at all points round the lens in relation to the outer edge. Where these two operations are omitted, as is the case when using conventional machines, difficulties will arise when the lenses are fitted to the frames; more exactly, the bevel fails to correspond to the curvature of the lens with sufficient precision, and the frames thus have to be significantly distorted in order to accommodate the lenses.
The main object of the invention disclosed is that of overcoming the drawbacks mentioned above, providing a machine that will supply the cutter to match the required bevel, automatically.
A further object of the invention is that of enabling adjustment of the position of the bevel in relation to the outer edge of the lens, maintaining the position constant at all points around the lens, notwithstanding variations in curvature.
An additional object of the invention is that of enabling adjustment of the rate at which the lens carriage is fed into the cut, and control of the speed at which the various cuts are implemented, according to the hardness of the material from which the lens is fashioned.